(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) and a driving method thereof. More specifically, the present invention relates to an LCD panel, an LCD including the same and a method for driving the same to achieve a high-speed response.
(b) Description of the Related Art
As personal computers (PC) and television sets have become lighter in weight and slimmer in thickness, display devices have also been required to become lighter and slimmer, and flat panel displays such as the LCD instead of cathode ray tubes (CRT) have been developed and used in practice to meet such requirements.
In the LCD, an electric field is used to arrange liquid crystal material that has anisotropic permittivity and is provided between two substrates. By adjusting the strength of the electric field, lights that transmits the substrates are adjusted. Accordingly, desired image signals are obtained. The LCD is one of the most used portable flat panel displays. Particularly, thin film transistor (TFT) LCDs using TFTs as switching elements are widely used.
FIG. 1 shows a pixel equivalent circuit of a general TFT-LCD.
As shown, the pixel of the general TFT-LCD comprises a TFT switching element that has a source electrode coupled to a data line and a gate electrode to a gate line; a liquid crystal capacitor Clc coupled to the drain electrode of the TFT switching element; a storage capacitor Cst coupled to the drain electrode of the TFT switching element; a parasitic capacitor Cgd provided between the gate and drain electrodes of the TFT switching element; a parasitic capacitor Cds provided between the drain and source electrodes of the TFT switching element; and an overlap capacitor Cover provided between the data line and a pixel electrode.
An operation of the liquid crystal provided between the pixel electrode on a TFT substrate and a common electrode on a color filter substrate will now be described.
First, when a bipolar pulse is supplied via the gate line, the TFT switching element is turned on. At this time, a signal voltage supplied to the source electrode of the TFT switching element via a signal line is supplied to a liquid crystal capacitor and a storage capacitor via the drain electrode. The signal voltage supplied together with the gate pulse is maintained by the storage capacitor and supplied to the liquid crystal capacitor after the gate voltage is turned off.
According to the above-described method for manufacturing the storage capacitor, the TFT-LCD is categorized as a previous gate method (or an additional capacitance method) as shown in FIG. 2a, and a common method (or an individual wiring method) as shown in FIG. 2b. 
As shown, the previous gate method uses a capacitor provided between a pixel electrode and a previous gate as a storage capacitor, and the common method generates a storage electrode in the pixel electrode and uses a capacitor between the storage electrode and the pixel electrode as the storage capacitor. The storage electrode of the common method is connected to a transparent common electrode line of the color filter substrate and is then driven.
When using an LCD to big screen applications, the biggest restriction is the response time. For a big size LCD, this invention is directed to a method for improving the response speed of the LCD using the previous gate method.
FIG. 3 shows a pixel equivalent circuit of the TFT-LCD using the previous gate, and FIG. 4 shows waveforms for describing the improvement of the response speed using the previous gate of FIG. 3.
As shown in FIG. 3, in the pixel equivalent circuit of the TFT-LCD, one terminal of the storage capacitor Cst is connected to the drain electrode and another terminal to the previous gate.
In operation, a predetermined switching pulse signal is supplied to the gate line, and the voltage finally supplied to the pixel by the common electrode voltage is as follows:
                              V          p                =                              ±                          V              s                                +                                                                      C                  st                                                                      C                    st                                    +                                                            C                      gd                                        ⁢                                          C                      lc                                                                                  ·              Δ                        ⁢                                                  ⁢                          V              g                                                          Equation        ⁢                                  ⁢        1            
where Vs represents the voltage supplied to the source electrode, Cst represents the capacitance of the storage capacitor, Cgd represents the parasitic capacitance between the gate and drain electrodes, Clc represents the capacitance of the liquid crystal capacitor, and ΔVg represents the difference voltage between the previous gate voltage and the present gate voltage.
However, since the above-noted method uses the previous gate structure, a heavy gate load is generated. Also, since the method can only be applied to line inversion driving, cross-talk and flickers are generated and it is difficult to achieve a high degree of precision.
Also, conventional gate TAP-ICs (Tape Aided Bonding-Integrated Circuits) cannot be used, and if the gate voltage at an off state is heavily increased, the off state current Ioff becomes great. And accordingly, gate value modification is limited.